This invention relates to detecting and collecting mercury for air or other gas, and particularly is concerned with an apparatus for and a method of mercury speciation, which can differentiate between different components of gaseous mercury. More particularly the inventor is concerned with differentiating between the different components of mercury in ambient air and stack gas, namely gaseous elemental mercury vapour, gaseous ionic water soluble compounds of mercury, and particulate bound mercury.
This invention is concerned with the detection of mercury and speciation of mercury, both in ambient air and in stack gases. There are three basic or largest components of gaseous or airborne mercury, namely: gaseous elemental mercury vapour, gaseous ionic water soluble compounds of mercury, and particulate bound mercury. Of the three basic components of gaseous or airborne mercury, the largest is gaseous elemental mercury Hg0, i.e. non-ionized mercury vapour. Gaseous ionic water soluble compounds of mercury are generally known by the collective designations: reactive gaseous mercury (RGM), ionic mercury, or Hg (II), or Hg2+; this class of compounds consists almost exclusively of mercury chloride, HgCl2, since this compound is produced by combustion processes that have free chlorine present (for example, coal-fired power plants, waste incinerators, etc.). Particulate bound mercury comprises particles having mercury bound to them. In ambient air, the large majority of mercury in particulate form is contained in small particles  less than 2.5 xcexcm (microns) in diameter.
In background ambient air, elemental mercury generally comprises 90-98% of the total mercury; in stack gases, the proportions tend to be more equal. However, even for ambient air, the small reactive gaseous portion is extremely important, since it deposits locally and, being water soluble is a much larger local concern. Elemental mercury, which generally has a lifetime in the atmosphere 6-12 months, will often be deposited well away from its source. The RGM is present at very low concentration, e.g. 10-50 pg/m3 so it must be preconcentrated before being analyzed.
There have been proposals in the art for measuring mercury and providing some degree of mercury speciation, i.e. measuring separately two or more components of mercury. Thus, one proposal is found in the paper entitled xe2x80x9cAtmospheric Mercury Speciation: Concentrations and Behaviour of Reactive Gases, Mercury in Ambient Airxe2x80x9d by S. E. Lindberg et al; Environmental Science and Technology 1998, Vol. 32, No. 1, pp. 49-57. There, the proposal is to use a high-flow refluxing mist chamber. This operates by drawing the sampled air through water disbursed as a fine aerosol. Water-soluble gases are adsorbed by the nebulised mist, and the droplets containing the scrubbed reactive gases of mercury coalesce on the surface of a hydrophobic membrane and then drain back into the chamber. The small solution volume required enables sampling times of the order of one hour to be used. Simultaneously, total gaseous mercury can be collected on old-coated sand adsorbers, but it should be noted that this technique collects all gaseous mercury, including the RGM. Particulate mercury is trapped by an external filter, which also traps some of the RGM. The mist chamber also has a particulate filter. The use of these filters tend to bias some of the RGM results low. The solutions from the mist chamber were analyzed by a wet chemistry technique, requiring reduction of the water soluble mercury ions to Hg0 by stannous chloride. The mercury is purged from the mixture into a carrier gas using standard techniques and preconcentrated onto a gold cartridge. The total gaseous mercury was detected on the gold adsorber in known manner using cold vapour atomic fluorescence. What is noteworthy about this technique is that all the mercury species are not determined together, but require separate detection techniques, and moreover due to the wet chemistry techniques required to analyze the collected RGM, the technique is neither continuous nor suitable for automation.
A further proposal is found in the paper entitled xe2x80x9cSampling and Determination of Gas Phase Divalent Mercury in the Air using a KCI Coated Denuderxe2x80x9d by Z. Xiao et al. in Fresenius Journal of Analytic Chemistry (1997) 358: 386-391. The technique proposed there used KCl denuders, formed by coating appropriately dimensioned glass tubes with KCl from a saturated KCl solution. The experiments reported show that gaseous elemental mercury (Hg0), simply passes through the denuders without being adsorbed, and apart from some apparent system errors, 100% xe2x80x9cpassagexe2x80x9d of elemental mercury vapour was reported. For field sampling, two lines were set up, one for collecting gaseous Hg2+ species using a KCl denuder and the other for conventional sampling of total gaseous mercury. It is noted that the effective recovery and analysis of the collected mercury from a denuder is an important step. Here, the mercury is recovered by HCl extraction followed by CVAFS (Cooled Vapour Atomic Fluorescent Spectroscopy). Again, such a technique is complex, and is not suited for continuous monitoring and cannot readily be automated.
A further series of developments and experiments have been reported by K. Larjava and others as follows:
xe2x80x9cDevelopment and Laboratory Investigations of a Denuder Sampling System for the Determination of Heavy Metal Species in Flue Gases at Elevated Temperaturesxe2x80x9d by K. Larjava et al, International Journal of Environmental and Analytical Chemistry, 38 (1990), pp. 31-45;
Application of the Diffusion Screen Technique to the Determination of Gaseous Mercury and Mercury (II) Chloride in Flue Gases by K. Larjava et al., accepted for publication in the International Journal of Environmental and Analytical Chemistry; and
On the measurement of Volatile Metal Species at Elevated Temperatures by K. Larjavaxe2x80x94Dissertation for the degree of Doctor of Technology to be presented at Helsinki University of Technology on May 21, 1993.
Here again, there is a disclosure of the use of annular denuder tubes coated with potassium chloride for collecting mercury species. These papers focus on the basic research and do not teach any complete, functioning instrument, capable of speciating mercury vapour.
Accordingly, known techniques suffer from a number of disadvantages. Commonly, they require a denuder collector or the like to be extracted from the equipment and analyzed using wet chemical techniques. Such a method is not suited to continuous monitoring, nor to an automated system.
A further problem with detection of RGM, which has not been fully identified in the art, is that common ionic mercury compounds have a strong affinity to adsorb onto a wide variety of surfaces. In this sense, ionic mercury is xe2x80x9cstickyxe2x80x9d, and extremely difficult to handle. Bearing in mind that the problem is to detect very low levels of ionic mercury, the small amount present readily adsorbs onto surfaces of tubing, valves and other fittings distorting any reading made.
Another problem with the determination of RGM is that it is very difficult to separate the RGM from the particulate mercury. Previously, this has been extremely difficult, as one known technique is to provide a particulate filter upstream of a denuder, to filter out the particulate mercury. However, due to the characteristics of reactive gaseous mercury, some fraction of it would collect on the particulate filters, leading to a false low reading for the measured amount of reactive gaseous mercury.
The present inventors have realized that, when measuring mercury vapour, including reactive gaseous mercury, it is necessary to take extra steps, to ensure that the reactive gaseous mercury does not collect onto surfaces before reaching a detection device intended to detect its presence.
Additionally, the present invention is directed to a technique which will allow continuous and automated measurement of mercury vapour, including speciating of the three principal components, namely gaseous elemental mercury vapour, gaseous ionic water soluble compounds of mercury or reactive gaseous mercury, and particulate bound mercury. More particularly, the present invention proposes detecting these three elements in a particular sequence, using appropriate detection techniques which will not interfere with one another.
The present invention is also directed to providing an improvement in the analysis of samples collected in denuders or other adsorption units coated with potassium chloride, or other suitable coating for detecting mercury. More specifically, the present invention is intended to avoid the use of complex wet chemistry techniques, which are not suitable to automated operation.
In background ambient air, elemental mercury generally comprises 90-98% of the total mercury. However, the small remaining reactive portion is extremely important, since it deposits locally and, being water soluble is a much larger local concern than elemental mercury, which generally has a lifetime in the atmosphere of 6-12 months.
The reactive gaseous mercury is present in such low concentrations (the values are typically 10-50 pg/m3). It must be preconcentrated before being analyzed. The present invention provides a device that can be used as a front end or interface for a conventional mercury analyzer, through which air can be passed at a high flow rate, typically 10 litres per minute, for a varied period of time, typically 40 minutes to 2 hours to effect this preconcentration.
The present invention has numerous advantages over the prior art. Where a regeneratable particulate filter is provided, then the three principal components or gaseous mercury can be separated and analyzed. This can be carried out continuously and in an automated fashion. The present invention in preferred embodiments, provides a regeneratable particle filter located downstream of a denuder, so as not to interfere with collection of reactive gaseous mercury in the denuder.
In accordance with a first aspect of the present invention, there is provided a detection module, for detecting airborne reactive gaseous mercury and for use with mercury analysis equipment, the detection module comprising:
an adsorption unit having an extended transfer surface and a coating on the transfer surface for adsorbing reactive gaseous mercury from a gas passing through the adsorption unit, the adsorption unit having an adsorption unit inlet opening directly to a gas sample containing reactive gaseous mercury and an adsorption unit outlet for connection to a pump means, for drawing the gas sample through the adsorption unit;
a first inlet for a flushing gas, substantially free of mercury vapour, for flushing out of the adsorption unit and connectable to the adsorption unit inlet;
a heating means for heating the adsorption unit to a suitable desorption temperature higher than an adsorption temperature to cause reactive gaseous mercury adsorbed onto the coating to be released as gaseous elemental mercury vapour, for measurement; and
a temperature controller for controlling the heating means, whereby the adsorption unit can be maintained at the adsorption temperature for adsorption of the reactive gaseous mercury and, at the desorption temperature for desorption of elemental mercury.
The adsorption unit can be any suitable device which provides an extended surface for adsorption of RGM, and capable of releasing mercury by heating. A denuder configuration is preferred where particulate mercury is present, as this permits the particulate mercury to pass through, for collection in a filter. For other uses, particularly where particulate mercury is not a concern, other adsorption unit configurations can be provided, e.g. packed quartz chips.
Preferably, the heating means includes a first heating element located around the denuder and a first heating controller connected to the first heating element, the first heating controller being capable of regulating the temperature of the first heating element to a first temperature at which the denuder functions to adsorb the reactive gaseous mercury, and to a second, elevated temperature at which reactive gaseous mercury compounds adsorbed on the denuder break down and release mercury as gaseous elemental mercury vapour.
The apparatus advantageously includes a detection module housing, in which the denuder is mounted with the denuder inlet extending out of the detection module housing, and wherein the heating means includes a second heating element located around the denuder inlet and a second heater controller connected to the second heating element, for maintaining the denuder inlet at a set temperature. The heating means can include a third heating element within the detection module housing for heating the housing and a third heating element controller for control thereof.
Preferably, at least one fan is provided for cooling the denuder, and more preferably, there are a first fan for cooling the denuder and a second fan for blowing air through the detection module housing for cooling thereof.
To handle particulate mercury, a filter pack can be provided, preferably comprising a filter material capable of being subjected to elevated temperature to release of mercury vapour to regenerate the filter pack, whereby particulate mercury can be collected in the particulate filter pack and the mercury can be released as elemental mercury vapour by heating of the filter material.
The apparatus can include, independably of or in addition to other aspects of the invention, an inlet T-shaped connector connected to the denuder inlet, the inlet T-shaped connector providing a straight through connection for a gas sample and a side connection connected to the first inlet for supply of flushing gas. This is a significant aspect of the present invention as it eliminates the need for a valve at the denuder inlet. It is to be noted that the connector need not be T-shaped; the key concept is to use the flushing gas flow to close off the denuder inlet to the exterior and for this purpose the inlet can have various shapes provided it includes an opening for the sample gas and an opening for the flushing gas.
Conveniently, a first valve is provided having a first connection port connected to the first inlet, a first valve outlet and a second valve outlet, with the side connection of the T-shaped connection connected to second valve outlet, and a flow restrictor connected to the first valve outlet, whereby the first valve can selectively connect the first inlet to either the flow restrictor for enabling a restricted flow of flushing gas to vent to atmosphere or to the denuder through the T-shaped connection for supply of flushing gas.
Preferably, the denuder is of the annular type and the coating comprises a salt, or a combination of salts, more preferably potassium chloride. The denuder is preferably formed from quartz glass.
Another aspect of the present invention provides an apparatus for the detection of reactive gaseous mercury, the apparatus comprising:
(1) an adsorption unit having an extended surface and a coating of ionic salt on the extended surface for adsorbing reactive gaseous mercury, the adsorption unit having an adsorption unit inlet opening directly to a sample gas and an adsorption unit outlet;
(2) a heating means for heating the adsorption unit to a suitable desorption temperature higher than an adsorption temperature to cause gaseous reactive mercury adsorbed on the coating to be desorbed and vaporized as elemental mercury;
(3) a temperature controller for controlling the heating means, whereby the adsorption unit can be maintained at the adsorption temperature for adsorption of the reactive gaseous mercury and, at the desorption temperature for desorption;
(4) a pump means connected to the adsorption unit outlet for drawing a gas sample through the adsorption unit; and
(5) a flushing gas supply means for supply of a flushing gas and connectable to the adsorption unit inlet, for passing flushing gas through the adsorption unit, whereby in use, a sample gas is drawn through the adsorption unit by the pump means in a sampling phase and reactive gaseous mercury is adsorbed onto the coating of the adsorption unit, and in a desorption phase, the heating means is actuated and flushing gas is passed through the adsorption unit from the adsorption unit inlet to the adsorption unit outlet so that adsorbed reactive gaseous mercury is desorbed, vaporized and entrained in the flushing gas flow.
Conveniently, the flushing gas supply means is integral with the pump means and is connected to the denuder inlet by a flushing gas supply line.
The pump means can be connected to the denuder outlet by a sample line, and the sample line then includes a branch connection port for connection to an analyzer.
Another aspect of the present invention provides an apparatus for use in monitoring stack gases. The apparatus then includes a dilution unit having an inlet for stack gases, an outlet connected to the denuder outlet, and an inlet for dilution air. The dilution air inlet is preferably connected to the flushing gas supply means. Such an apparatus can include an acid gas scrubber connected between the pump means and the denuder outlet for scrubbing acid gas from the gas flow.
Another aspect of the present invention provides a method for determining the quantity of reactive gaseous mercury in a gas, the method comprising the steps of:
(1) passing a gas sample through an adsorption unit having an adsorption unit inlet opening directly to a gas sample and an adsorption unit outlet, said adsorption unit having an extended surface provided with a coating for adsorbing reactive gaseous mercury and being maintained at a suitable adsorption temperature above ambient temperature of the gas sample to prevent condensation of water vapour during adsorption;
(2) after a known quantity of gas has been passed through the adsorption unit, terminating supply of the sampled gas, and passing a flushing gas through the adsorption unit; and
(3) while passing the flushing gas through the adsorption unit, heating the adsorption unit to a suitable desorption temperature higher than the desorption temperature to cause desorption of reactive gaseous mercury compounds as gaseous elemental mercury vapour for entrainment in the flushing gas, and passing the flushing gas with the entrained gaseous elemental mercury vapour to a mercury analyzer.
Preferably, the denuder in step (3) is heated to a temperature in the range of 500xc2x0 C.
Preferably, the method includes passing a portion of the gas sample that has passed through the denuder to a mercury analyzer, for determination of the level of gaseous elemental mercury vapour in the gas sample.
Where particulate mercury might be present, the method can include filtering out particles containing particulate mercury from the gas sample, downstream from the denuder. This is preferably done with a regeneratable filter pack, and prior to step (3), the regeneratable filter pack is heated to a temperature whereby particulate mercury is desorbed from the regeneratable filter pack as gaseous elemental mercury vapour, and that gaseous elemental mercury vapour in is then entrained in the flushing gas flow, for determination of the level of particulate bound mercury.
In another aspect of the present invention an extract from stack gases is taken, and, prior to step (1), the extract is diluted with a flushing gas, substantially free of mercury and having a low relative humidity, to dilute the stack gas extract and to form the gas sample with a reduced relative humidity.